Autistic Disorder (AutD) is a neurodevelopmental disorder characterized by significant disturbances in social, communicative, and behavioral functioning. Onset of AutD is in early childhood with symptoms continuing throughout life. Prevalence estimates for AutD range from 2-10/10,000. Epidemiologic studies have consistently implicated genetic factors in the etiology of AutD with sibling recurrence risk ratio (lambdas) estimates ranging from 100-200. The consensus of recent studies suggests that there are anywhere from 2-10 loci underlying the genetic form of AutD. Because of the rapid decrease in lambdas with increasing degree of relationship in AutD, epistatic interactions of these multiple loci is suggested. We currently have independently funded research efforts on chromosomes 7 and 15, two regions indicated as potentially containing AutD risk loci based on a combination of cytogenetic and linkage evidence. To date there have been seven genome screens in autism, indicating several additional consensus regions of interest including chromosomes 2, 3, and 19. The etiology of AutD is now ripe for dissection using newly developed statistical and molecular genetic tools. Our long-range goal is to identify all major, moderate, and epistatic genetic effects in AutD. The isolation of the genes for AutD and related disorders such as Asperger's disease (AspD) and the dissection of how these genes interact will result in successful therapeutic interventions and is therefore of critical import. Specifically we propose to: 1) extend our AutD family collection and to begin collecting AspD trios; 2) examine genes homologous to the MECP2 as susceptibility genes in AutD; 3) Investigate consensus regions of linkage interest on chromosomes 2, 3 and 19; 4) develop and apply the MDR-PDT method in order to identify possible epistasis and non-additive gene-gene interaction effects in AutD.